The theory of surface -enhanced Raman spectroscopy on organic semiconductors: J -aggregates

Drawing on the theory of surface-enhanced Raman spectroscopy (SERS) in semiconductor substrates, we examine the surface-enhancement of the Raman signal when the substrate is chosen to be an organic J-Aggregate. The underlying theory involves breakdowns of the Born-Oppenheimer approximation, leading to vibronic coupling originally proposed by Herzberg and Teller. Since the theory of the energy levels of J-aggregates are well-known and clearly defined, we may integrate the vibronically coupled expression to obtain the contribution to the SERS enhancement (EF) due to the substrate aggregation. It is shown that this involves two effects: the square of the coherence length of the aggregate (N + 1) as well as the fourth power of the linewidth narrowing of the exciton band from (γmonomer)/(γJagg) such that: EF=[Formula presented] Thus, with the knowledge of the coherence length as well as the linewidth narrowing of the aggregate absorption, we can predict the contribution of J-aggregation to the SERS enhancement. © 2020